Fiber Orientation and Fracture Morphology in Short Fiber-Reinforced Thermoplastics

Spatial fiber distribution and fracture morphology were studied for three injection-molded thermoplastic composites: PPS, PAI, and PEEK reinforced with carbon fibers. Specimens were dog bone-shaped tensile coupons and flex bars. Both optical and scanning electron microscopes were used. In general, most fibers were found to be aligned in the mold-fill direction except in the core. The fibers in the core tended to be normal to the mold-fill direction. Yet, the exact distribution pattern varied from material to material and depended upon the gate design. Examinations of fracture surfaces indicated that the best interfacial bonding was in the PEEK composite followed by the PAI composite. In the core, fracture surfaces generally followed the fiber orientations. The PPS composite showed the most improvement over the matrix properties in both static and fatigue strengths despite poor interfacial bonding. The least improvement was observed of the PAI composite in which matrix cracks were seen to cut through fibers. Nevertheless, the strongest matrix, PAI, yielded the strongest composite, and the weakest matrix, PPS, yielded the weakest composite.